RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, annims, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the of the objects, items, animals, and people. It has been known since the early 60's in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components on a transponder could be powered by radio frequency (RF) power sent by a "base station" at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current would lead to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna can be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own, it is called a "passive" tag to distinguish it from an active tag containing a battery. The battery supplies energy to run the active tag electronics. An active tag may also change the loading on the tag antenna for the purpose of transmitting information to the base station, or it may act as a transmitter to broadcast the information from the tag antenna directly to the base station.
The "rebroadcast" of the incoming RF energy at the carrier frequency is conventionally called "back scattering", even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed "back" to the transmitting antenna.
In the 70's, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor.
Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station.
U.S. Pat. No. 5,214,410, hereby incorporated by reference, teaches a method for a base station to communicate with a plurality of Tags.
Prior art tags typically use a number of discrete components connected together with an antenna. However, to substantially reduce the cost of the tags, a single chip connected to an antenna must be used.
The environment of RF tags is such that the power transferred to the tags may vary because the tag is moving with respect to the base station or because other sources of interference are changing while the base station is trying to write information to the non-volatile tag memory. Typically, the information is written a single byte at a time. The tag voltage is assumed to be high enough to write to the E.sup.2 PROM memory elements during the entire time the byte is being written to memory. However, if the voltage is not high enough, not enough charge will be stored in the E.sup.2 PROM memory elements. Enough charge may be stored that the base station writing the memory can call for a read back of the memory that was just written, and the read back data will be correct. However, if the tag is moved to another location, the same or another base station may read the memory and get a different result if not enough charge is stored in the E.sup.2 PROM memory elements.